Random Variation

I can't really help you on the scripting part but what I have played with and used from time to time for animation purpose was the Real-World clock example.
Real World Clock

My thoughts:

I am pretty certain that the global VARIATION is a read-only variable and you can't change it with a script.

I can think of two ways to approach this. Both will only generate random grey values, so you will have to construct your filter accordingly. (Meaning, you can't affect grey control component inputs, but will have to construct a similar effect fr om a green component input.)

The first one is to query the system time with an unsafe script and generate the input values that way. I gather this is what you already tried. The lim itation with that approach is that you get a different value only if the filter is applied with an interval of more than 1 second.

The second one is to pick the value(s) from the input image. With video, you should have varying values for each frame. If you pick, say, the center point gray value, you get 256 variations or if you pick RGB values, you can get 256*256*256 different values, etc.